For the cooling of rooms, it has become common to provide the ceilings of the rooms with supply air devices, wherein incoming or primary air supplied from a central ventilation system is blown from nozzles inside the device at a relatively high rate to a mixing element inside the supply air device, where a circulating i.e. secondary air flow from the room is induced into the supply air jets. The air mixture formed by the primary and secondary air is led from the supply air device into the room. The secondary air induced from the room enters the supply air device via a heat exchanger which enables the cooling or, alternatively, the heating of the air. The provision of such supply air devices has resulted in a significant improvement in the control of internal thermal conditions. However, there has not been any corresponding development in the removal of impurities from indoor air.
In view of the purity of indoor air, for example, construction materials with a minimum of emissions and impurity-freer components of air-conditioning systems have been developed, and investments have been made on e.g. the regular cleaning of central air-conditioning systems and more efficient filtering of air. To improve the control of impurities in indoor air, another significant measure for the air-conditioning is to increase the outdoor air flow.
The significance of increasing the outdoor air flow rate is relatively limited in view of the quality of indoor air. For example, doubling the air flow rate will reduce the content of impurities from indoor sources in indoor air into a half. However, increasing the outdoor air flow rate will increase the need for cooling and heating and thereby contradict the aim of limiting energy consumption in buildings. Increasing the supply air flow rates will involve an increase in the size of the conditioned air ducts, which will result in an increase in the height of floors in buildings, thereby increasing the costs.
A filtering arrangement in the supply air device itself is presented, for example, in DE 3321612. The document discloses a supply air device, in which a secondary air flow is led through a filter before a heat exchanger. The filter used is a conventional filter mat which, however, causes a high flow resistance, which is not advantageous for the function of the supply air device. The high flow resistance reduces the secondary air flow too much, wherein the filtering of the secondary air flow does not have a significant effect on the quality of indoor air. If the supply air device is equipped with cooling or heating, the flow resistance of the filter mat restricts the air flow through said elements too much, resulting in an excessive reduction in the cooling/heating efficiency of the supply air device. If the filter used is a fabric filter that is very permeable to air, the secondary air flow may be at an acceptable level, but the efficiency of purification with the filter will remain low.